Nitride semiconductor device

ABSTRACT

A nitride semiconductor device includes: a main semiconductor region comprising a first nitride semiconductor layer having a first band gap, and a second nitride semiconductor layer having a second band gap larger than the first band gap, a heterojunction being formed between the first nitride semiconductor layer and the second nitride semiconductor layer such that a two-dimensional electron gas layer can be caused inside the first nitride semiconductor layer based on the heterojunction; a source electrode; a drain electrode; a third nitride semiconductor layer formed on the first nitride semiconductor layer and between the source electrode and the drain electrode; a fourth nitride semiconductor layer formed on the third nitride semiconductor layer and having p-type conductivity; and a gate electrode formed on the fourth nitride semiconductor layer. The third nitride semiconductor layer has a third band gap smaller than the first band gap.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims priority from Japanese PatentApplication No. 2008-275717 filed on Oct. 27, 2008, the entire contentsof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a high electron mobility transistor(HEMT) of a normally-off type (enhancement type) formed of a nitridesemiconductor device.

2. Description of the Related Art

A nitride semiconductor has an electric field drift speed and abreakdown electric field strength very higher than Si or GaAs. At aheterojunction interface occurring between nitride semiconductors, ahigh-density carrier layer called a two-dimensional carrier gas isformed by a polarization effect. From these features, for example, ahigh electron mobility transistor (HEMT) having an AlGaN/GaNheterojunction structure is applied as a switching device for powersource operating at a large current and a high voltage.

The HEMT using GaN (0001) plane has a negative gate threshold voltage,and has characteristics in which a current flows between a sourceelectrode and a drain electrode in a state in which a gate controlvoltage is not applied to a gate electrode, that is, characteristics ofa normally-on type (depletion type). However, a switching device isdesired to be a normally-off type (enhancement type) having a positivegate threshold voltage in order to ensure safety at the time ofabnormality.

JP-A-2007-165719 describes a HEMT in which a channel layer made of GaNis formed on a barrier layer made of AlGaN in a GaN/AlGaNheterojunction. According to the HEMT, a lattice constant of the channellayer is larger than a lattice constant of the barrier layer, so thatpiezo polarization and spontaneous polarization occurring in the channellayer are canceled and thereby a carrier concentration of atwo-dimensional electron gas (2DEG) formed in the channel layer can bereduced and the HEMT having characteristics of a normally-off type canbe obtained.

SUMMARY OF THE INVENTION

However, in the above-described HEMT, from its structure, a distancebetween a gate electrode and a drain electrode becomes equal to a filmthickness of an insulator layer formed on a surface of the gateelectrode, so that the distance is small and hence high resistance tovoltage could not be obtained. Also, an increase in gate capacity by anincrease in gate length causes a decrease in high-frequencycharacteristics or a decrease in switching speed of the HEMT. Fromthese, the above-described HEMT may not be suitable for a high-frequencydevice or a power device.

An object of the present invention is to provide a normally-off typeHEMT having enhanced high-frequency characteristics, high switchingspeed and high resistance to voltage.

According to an aspect of the invention, there is provided a nitridesemiconductor device comprising: a main semiconductor region comprisinga first nitride semiconductor layer having a first band gap, and asecond nitride semiconductor layer having a second band gap larger thanthe first band gap, a heterojunction being formed between the firstnitride semiconductor layer and the second nitride semiconductor layersuch that a two-dimensional electron gas layer can be caused inside thefirst nitride semiconductor layer based on the heterojunction; a sourceelectrode formed on the main semiconductor region; a drain electrodeformed on the main semiconductor region and separated from the sourceelectrode; a third nitride semiconductor layer formed on the firstnitride semiconductor layer and between the source electrode and thedrain electrode; a fourth nitride semiconductor layer formed on thethird nitride semiconductor layer and having p-type conductivity; and agate electrode formed on the fourth nitride semiconductor layer, whereinthe third nitride semiconductor layer has a third band gap smaller thanthe first band gap.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a HEMT according to a firstembodiment of the invention;

FIG. 2 is a cross-sectional view of a HEMT according to a modifiedexample of the first embodiment of the invention;

FIG. 3 is a cross-sectional view of a HEMT according to a modifiedexample of the first embodiment of the invention; and

FIG. 4 is a structure sectional view of a HEMT according to a secondembodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A HEMT according to an embodiment of the invention will be describedwith reference to the drawings.

A HEMT 1 a according to a first embodiment of the invention shown inFIG. 1 includes a channel layer 2 (an example of a first nitridesemiconductor layer) formed on a single-crystal silicon semiconductorsubstrate (not shown), a barrier layer 3 (an example of a second nitridesemiconductor) formed on the channel layer 2, a source electrode 5formed on the barrier layer 3, a drain electrode 6 formed on the barrierlayer 3 and separated from the source electrode 5, a first gate layer 10(an example of a third nitride semiconductor layer) formed so as to abuton the channel layer 2 and extend through the barrier layer 3 betweenthe drain electrode 6 and the source electrode 5 on the channel layer 2,a second gate layer 11 (an example of a fourth nitride semiconductorlayer) formed on the first gate layer 10, and a gate electrode 7 formedon the second gate layer 11. In other words, combination of the channellayer 2 and the barrier layer 3 can be an example of a mainsemiconductor region.

The channel layer 2 is formed, for example, of Al_(x)Ga_(1-x)N (0<x≦1)and is formed on a single-crystal semiconductor substrate (not shown)using an epitaxial growth method such as a MOCVD method. In theembodiment, the channel layer is obtained by forming anAl_(0.3)Ga_(0.7)N layer of 0.5 to 2.0 μm. Also, the barrier layer 3 isformed of a nitride semiconductor material having a band gap larger thanthat of the channel layer 2 and is formed by Al_(y)Ga_(1-y)N (0<y≦1)x<y) in the embodiment. The barrier layer 3 is formed by increasing anAl composition ratio by a technique similar to the channel layer 2. Inthe embodiment, the barrier layer is obtained by forming anAl_(0.5)Ga_(0.5)N layer of 20 nm. Since a band gap of the channel layer2 is larger than that of the barrier layer 3, a two-dimensional electrongas layer (2DEG layer) 4 by piezo polarization and spontaneouspolarization is formed in the channel layer 2 based on a heterojunctionformed by the channel layer 2 and the barrier layer 3. Since an electricfield direction based on this piezo polarization is equal to an electricfield direction based on the spontaneous polarization of the channellayer 2, a carrier concentration of the 2DEG layer 4 higher than that ofa conventional HEMT is obtained.

The first gate layer 10 is formed of a nitride semiconductor materialhaving a band gap smaller than that of the channel layer 2 and is formedby GaN in the embodiment. The first gate layer 10 is obtained by:forming a mask so as to have an opening in a position in which the firstgate layer 10 is formed in the barrier layer 3; performing dry etchingsuch as RIE so as to form an opening part reaching the channel layer 2;and thereafter forming a non-doped GaN layer of 10 to 50 nm by theepitaxial growth method such as the MOCVD method. As used herein, theterm “non-doped” means that semiconductor impurities are not addedintentionally. The first gate layer 10 grows so as to be lattice-matchedto AlGaN of the channel layer 2 and has a band gap smaller than that ofthe channel layer 2, so that piezo polarization by compressive stressaffects the first gate layer 10 based on a heterojunction formed by thechannel layer 2 and the first gate layer 10. That is, from a local pointof view, it can be rephrased that the channel layer 2 acts as thebarrier layer and the first gate layer 10 acts as the channel layer.Since an electric field direction based on this piezo polarizationbecomes a direction opposite to an electric field direction based onspontaneous polarization of the first gate layer 10, the 2DEG layerbecomes resistant to being formed in the vicinity of a junctioninterface between the first gate layer 10 and the channel layer 2 in thefirst gate layer 10.

In addition to the manufacturing method described above, the barrierlayer 3 may be selectively grown on the channel layer 2 after anon-doped GaN layer is grown on the whole surface of the channel layer 2and dry etching is performed except for a position in which the firstgate layer 10 is formed. Also, the first gate layer 10 may be formed byAlGaN with an Al composition ratio smaller than that of the channellayer 2.

The second gate layer 11 includes GaN doped with p-type impurities suchas magnesium (Mg), and is formed by performing epitaxial growthcontinuously using the MOCVD method following a process of forming thefirst gate layer 10. The second gate layer is formed so that a thicknessof the second gate layer 11 in the embodiment is 100 nm and its carrier(hole) concentration becomes 1×10⁻¹⁸ to 1×10⁻¹⁹ cm⁻³.

The source electrode 5, the drain electrode 6 and the gate electrode 7are formed of a laminated structure including, for example, aluminum(Al) and gold (Au), and are formed by patterning by a photolithographystep after evaporated on the barrier layer 3 and the second gate layer11. The source electrode 5 and the drain electrode 6 are formed on thebarrier layer 3, and a thickness of the barrier layer 3 is very thin anda resistance of a thickness direction is extremely low. Therefore, it issaid that the source electrode 5 and the drain electrode 6 areelectrically connected to the 2DEG layer 4 through the barrier layer 3.Also, the gate electrode 7 is formed so as to be electrically connectedto the second gate layer 11. Although an illustration is omitted, thegate electrode 7 may be formed by a field plate structure extending onthe barrier layer 3 through an oxide film disposed on the barrier layer3.

Next, an operation of the HEMT 1 a according to the embodiment will bedescribed.

The first gate layer 10 grows so as to be lattice-matched to AlGaN ofthe channel layer 2, and a band gap is smaller than that of the channellayer 2. Therefore, it is difficult to occur 2DEG based on aheterojunction between the channel layer 2 and the first gate layer 10.Further, the second gate layer 11 has p-type conductivity, so that a pnjunction is formed between the first gate layer 10 and the second gatelayer 11 and thereby, the first gate layer 10 is completely depleted bya depletion layer extending from a pn junction interface andnormally-off characteristics can be increased.

As a result of that, a current path is divided in the first gate layer10 in a state in which a positive gate control voltage is not applied tothe gate electrode 7. Therefore, a current does not flow in the HEMT 1 aeven when a potential of the drain electrode 6 is made higher than apotential of the source electrode 5.

On the other hand, when the positive gate control voltage is applied tothe gate electrode 7, a pn junction formed by the first gate layer 10and the second gate layer 11 is forward biased, so that a carrier isgenerated in the first gate layer 10 and a channel resulting in acurrent path is formed. When a potential of the drain electrode 6 ismade higher than a potential of the source electrode 5 in this state, adrain current flows between the drain electrode 6 and the sourceelectrode 5 via the channel in the first gate layer 10 and the 2DEGlayer 4. That is, a nitride semiconductor device having normally-offcharacteristics is obtained.

Further, according to the HEMT 1 a according to the embodiment, highresistance to voltage can be achieved easily by separating the gateelectrode 7 from the drain electrode 6. In other words, the distancebetween the gate electrode and the drain electrode 6 can be increased,for example, as compared with the HEMT described in the background art.Further, a gate length does not increase, so that a high switching speedis obtained. Also, an electric field direction based on spontaneouspolarization of the channel layer 2 is equal to an electric fielddirection based on piezo polarization based on a heterojunction betweenthe channel layer 2 and the barrier layer 3, so that a carrierconcentration of the 2DEG layer 4 can easily be made higher than that ofa HEMT described in the background art, and an ON resistance of the HEMT1 a can be reduced. Therefore, a high-frequency device having developedhigh-frequency characteristics or a power device with a high switchingspeed capable of operating at a large current and high resistance tovoltage is obtained.

Here, in the HEMT la according to the first embodiment of the invention,a source electrode 5 and a drain electrode 6 may be formed so as to abuton a 2DEG layer 4 as shown in FIG. 2.

Opening parts in which the source electrode 5 and the drain electrode 6are formed can be formed simultaneously or separately with an openingpart in which a first gate layer 10 is formed in the HEMT 1 a, and aftera second gate layer 11 is formed, the electrodes can be formed bypatterning by a photolithography process after evaporated on a barrierlayer 3 and the second gate layer 11.

According to a HEMT 1 b formed thus, an effect similar to the HEMT 1 aaccording to the first embodiment of the invention is obtained. Also, anelectrical resistance value between the source electrode 5 and the drainelectrode 6 and a 2DEG layer 4 can be decreased extremely and an ONresistance of the HEMT 1 b can be reduced.

Also, as shown in FIG. 3, a recess 13 having a side surface 12 may beformed in a barrier layer 3. Further, a first gate layer 10, a secondgate layer 11 and a gate electrode 7 may be formed in a state ofextending on the barrier layer 3 including the side surface 12 and achannel layer 2. The recess 13 having the side surface 12 can be formedby forming a mask so as to have a predetermined opening in the barrierlayer 3 and performing dry etching such as RIE. In addition, the sidesurface 12 may be formed so that a thickness of the barrier layer 3changes gradually or changes stepwise.

The first gate layer 10 and the second gate layer 11 are formedcontinuously by an epitaxial growth method such as an MOCVD method afterthe recess 13 is formed on a main semiconductor region.

According to a HEMT 1 c according to this example, an effect similar tothe HEMT 1 a according to the first embodiment can be obtained. Also,the first gate layer 10, the second gate layer 11 and the gate electrode7 are formed so as to extend on the barrier layer 3 and the channellayer 2, so that a reduction in convergence of an electric field by afield plate effect can well be achieved and higher resistance to voltagecan be achieved.

Next, a HEMT 1 d according to a second embodiment of the invention shownin FIG. 4 will be described. In FIG. 4, the description is omitted byassigning the same numerals to substantially the same elements as FIG.1.

The HEMT 1 d according to the present embodiment differs from the firstembodiment in that a first gate layer 10 a has an n-type conductivitytype, and others are formed similarly.

The first gate layer 10 a includes GaN doped with n-type impurities suchas silicon (Si), and is formed by adding dichlorosilane (SiH₂Cl₂) etc.to a raw material gas and performing an MOCVD method in the process offorming the first gate layer 10 in the first embodiment of theinvention. The first gate layer 10 a is formed so that a thickness ofthe first gate layer 10 a in the embodiment is 50 nm and its carrier(electron) concentration becomes 1×10⁻¹⁷ to 1×10⁻¹⁹ cm⁻³. Also, athickness of a second gate layer 11 is 50 nm and its carrier (hole)concentration is formed in 1×10⁻¹⁷ to 1×10⁻¹⁹ cm⁻³.

According to the HEMT 1 d according to this embodiment, an effectsimilar to the HEMT 1 a according to the first embodiment of theinvention is obtained. Also, the first gate layer 10 a has n-typeconductivity, so that an ON resistance can be reduced by forming achannel having a high carrier concentration when a positive gate controlvoltage is applied.

One example of the embodiments of the invention has been describedabove, but the invention is not limited to such specific embodiments,and various modifications and changes can be made within the scope ofthe invention in the claims. Also, each of the embodiments or each ofthe modified examples may be combined. For example, the second gatelayer 11 may be formed by AlGaN having substantially the same Alcomposition ratio as that of the first gate layer 10 or 10 a, and mayalso be formed by a different material. Also, each of the nitridesemiconductor layers may be formed by a material represented byAl_(x)In_(y)Ga_(1-x-y)N (0≦x<1, 0≦y≦1, 0≦x+y≦1) including indium (In),and an Al or In composition ratio and/or a thickness of each of thesemiconductor layers may be changed. Also, a passivation layer includingSiO may be formed on the barrier layer 3.

According to the embodiments of the invention, a normally-off type HEMThaving enhanced high-frequency characteristics, high switching speed andhigh resistance to voltage can be obtained.

1. A nitride semiconductor device comprising: a main semiconductorregion comprising a first nitride semiconductor layer having a firstband gap, and a second nitride semiconductor layer having a second bandgap larger than the first band gap, a heterojunction being formedbetween the first nitride semiconductor layer and the second nitridesemiconductor layer such that a two-dimensional electron gas layer canbe caused inside the first nitride semiconductor layer based on theheterojunction; a source electrode formed on the main semiconductorregion; a drain electrode formed on the main semiconductor region andseparated from the source electrode; a third nitride semiconductor layerformed on the first nitride semiconductor layer and between the sourceelectrode and the drain electrode; a fourth nitride semiconductor layerformed on the third nitride semiconductor layer and having p-typeconductivity; and a gate electrode formed on the fourth nitridesemiconductor layer, wherein the third nitride semiconductor layer has athird band gap smaller than the first band gap.
 2. The nitridesemiconductor device according to claim 1, wherein the third nitridesemiconductor layer is a non-doped semiconductor layer.
 3. The nitridesemiconductor device according to claim 1, wherein the third nitridesemiconductor layer has n-type conductivity.
 4. The nitridesemiconductor device according to claim 1, wherein the third nitridesemiconductor layer is formed of Al_(x)In_(y)Ga_(1-x-y)N (0≦x<1, 0≦y≦1,0≦x+y≦1).